It is known that carbon dioxide, methane and other relatively highly adsorbable gas components (hereinafter also referred to as product gas components) can be separated from raw gas mixtures containing them in association with less readily adsorbable gas components, also referred to hereinafter as waste gas components, upon suitable adsorbers in appropriate beds. The adsorbers which are most commonly used are those which contain carbon, i.e. are active carbons.
In the pressure swing adsorption system, each adsorber is passed through a cycle in which the adsorber bed is charged with the highly adsorbable components in an adsorption phase at a first pressure. The bed is then flushed in a flushing phase with a flushing gas, and the product gas components are then desorbed by reducing the pressure in the adsorber to a desorption pressure during a desorption phase.
The pressure swings to which each adsorber may be subjected are generated at least in part by connecting one adsorber to another adsorber at the pressure which is desired in the first and allowing the two interconnected adsorbers to reach an equilibrium.
Until now it has been difficult, if not impossible, to recover these adsorbed product gas components in a pure form, i.e. in a concentration in excess of 99.5% by vol. by the desorption step in a satisfactory yield.
In various technological applications, these product gas components are available at concentrations between 15 and 50% by vol. in gas mixtures which makes their recovery feasible, especially if the product gas which is recovered can contain them in concentrations of 99.5% by vol. or more. Thus, it is advantageous to recover methane from mine gases, acetylene from acetylene/hydrogen circulating gases, carbon monoxide from a reformer gas and carbon dioxide from a converter or flue gases.
The invention is applicable to all of these separations and indeed any separation in which methane or carbon dioxide is to be recovered in high yield and in high concentration from a raw gas mixture.
We have discovered that one of the problems with earlier methods is that during the desorption stage, concurrently adsorbed other gas components, i.e. gas components other than the highly adsorbable desired product gas components, may be liberated to form impurities in the product gas mixtures which are similar to the impurities which may be present in the starting or raw gas mixture.
This problem has been attacked by a method which, as set forth in Ser. No. 504,251, which is not a reference in the present case, and which comprises steps (a)-(d) as outlined below.